/Engine/Source/Editor/UnrealEd/Classes/ThumbnailRendering/ThumbnailRenderer.h
Checks to see if the specified asset supports realtime thumbnails, which will cause them to always be rerendered to reflect any changes made to the asset. If this is false, thumbnails should render once and then not update again. For most renderers, this should remain as true.
True if the thumbnail needs to always be redrawn, false if it can be just drawn once and then reused.
virtual bool AllowsRealtimeThumbnails ( [UObject](API\Runtime\CoreUObject\UObject\UObject) * Object ) const